This application claims the priority of German application 198 31 071.4, filed in Germany on Jul. 10, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a steering system for motor vehicles, a steering handle, for example, steering handwheel, capable of being operated by the driver, a steering actuating drive for the steering adjustment of steerable vehicle wheels, with a steering-angle desired-value encoder capable of being operated by the steering handle, a steering-angle actual-value encoder capable of being operated by the steerable vehicle wheels, and a regulating and control arrangement operating the steering actuating drive as a function of a comparison of the desired and actual values of the steering angle.
A steering system of the foregoing type may be designed, in particular, for steer-by-wire operation, in which, where appropriate, a mechanical steering column, customary in conventional steering systems, can be replaced by another mechanical system or else by a hydraulic system, in particular a hydrostatic system.
Car manufacturers are making great efforts, based on various concepts, to increase vehicle safety and, consequently, the safety of vehicle occupants. According to one of these concepts, measures are indicated, by means of which, for example, the risk of collision of the vehicle with an obstacle is reduced. Collisions of this kind may occur, for example, when, if a first vehicle changes lane, a second vehicle is in the so-called "blind spot" of the first vehicle. Collisions when the vehicle is being manoeuvred are less serious, but are all the more frequent. Also, the fear of collisions, for example when the vehicle is being parked, often means that, when parking spaces are tight, drivers often do not even attempt to park the vehicle.
Electronic parking aids are known for manoeuvring the vehicle. These aids generate an acoustic signal, and warn the driver, on the basis of range-measuring sensor technology, when a critical distance between the vehicle and an obstacle is reached. At higher speeds, parking aids of this type are unsuitable for avoiding collisions, for example during a change of lane.
EP 0,738,647 A2 discloses a conventional power steering system with a rigid steering column which has a servomotor for assisting a manual steering force exerted by the driver. Moreover, the known power steering system has means for deciding whether there is the possibility of a collision between the vehicle and an obstacle detected in the steering direction. When the risk of a collision is detected in the known power steering system, there is a reduction in the power assistance, so that a collision course can be controlled only by increased manual steering force. At the same time, in order to avoid a collision, the known power steering system may work as sluggishly as a conventional power steering system in which the power assistance has failed.
The collision avoidance principle used in the known power steering system can be applied, however, only to a steering system having a rigid steering column. It is not possible to use it in a steering system which works by steer-by-wire operation or in a steering system with an open steering column, that is to say a steering system without a rigid steering column, for example a steering system with a hydraulic rod, a steering column with couplings, planetary assemblies or other gearings which interrupt the rigid drive-through from the steering wheel to the steered wheels. This is because, in steering systems of this type without a rigid coupling between the steering handle and the steered wheels, such a reduction in the power assistance would not be noticed on the steering handwheel by the driver. Thus, if lateral wheel forces act on the steered wheels, changes in direction of travel which can no longer be controlled by the driver may occur.